1. Field of the Invention
This invention relates to a continuous process for the production of poly-2-pyrrolidone by the in situ production of catalysts for the polymerization of 2-pyrrolidone, polymerization under agitation and the isolation of the particulate polypyrrolidone product. Poly-2-pyrrolidone is produced by the alkaline catalyzed polymerization of 2-pyrrolidone. The catalyzed polymerization is initiated by a "dimer" formed by the condensation of two 2-pyrrolidone molecules or by other initiators such as N-acyl pyrrolidone. The dimer is systematically designated 1-(1-pyrrolin-2-yl)-2-pyrrolidinone. It is more conveniently called pyrrolinyl-pyrrolidone, or "dimer". Dimer is present in commercially available 2-pyrrolidone in greater or lesser amounts. It is removed by fractional distillation with great difficulty or not at all.
Polypyrrolidone has heretofore been produced by batch bulk polymerization. The monomer and catalysts were charged to a reactor and held at polymerization temperature for 20 or more hours to obtain a hard block of polymer. Since, in general, only 30-70 percent of the monomer was converted to polymer, the product had to be chopped, ground and extracted with water to recover unreacted monomer. The final polymer particle size depended on the extent of grinding, as well as the percent conversion to polymer. Polymerization under agitation, such as in a stirred reactor, produces a product which is a powder or alkaline paste or slurry of particulate polypyrrolidone and polymerizate. At conversions above about 40 percent, the product is powdery. The product is conveniently washed with water to provide a particulate polypyrrolidone without grinding. However, a considerable amount of polymer degradation occurs if the alkaline product is contacted with water while still at the elevated temperature of the polymerization reactor. If, on the other hand, the alkaline product is allowed to cool before contacting with water, it becomes a hard block of polymer requiring the same processing as the batch bulk polymerization product.
2. Prior Art
In U.S. Pat. No. 3,184,476 pyrrolinyl-pyrrolidone is removed from 2-pyrrolidone by heating with water for several hours at a temperature greater than 80.degree. C. Slight acidification of the 2-pyrrolidone was found to accelerate the hydrolysis of the dimer. On the other hand, alkaline hydrolysis of the sensitive 2-pyrrolidone ring is said to occur even under mild conditions of prolonged heating of aqueous alkaline 2-pyrrolidone (see U.S. Pat. Nos. 3,778,402 and 3,681,293). U.S. Pat. No. 3,213,066 discloses the production of granular polypyrrolidone by polymerization in a ball mill reactor. In the continuous polymerization process of U.S. Pat. No. 3,681,293 the polymerizate is extruded, and pelletized before drying and washing.